1. Field of the Invention
The invention is directed to a trip unit and a circuit breaker incorporating such a trip unit that is calibrated by adjusting a gap between a magnetic armature and magnetic yoke to which the armature is attracted by a selectable level of current in the protected circuit. The gap is adjusted without affecting the adjustable spring force that sets the current level at which the circuit breaker trips.
2. Background Information
Many circuit breakers utilize a magnetic trip unit to provide an xe2x80x9cinstantaneousxe2x80x9d response to overcurrent or short circuit conditions. In one common type of magnetic trip unit, a magnetic yoke positioned around the load conductor focuses the magnetic field induced by the load current to attract an armature, which as it moves toward the magnetic yoke actuates the operating mechanism that opens the circuit breaker""s separable contacts. The armature is biased away from the magnetic yoke by a spring. The spring force, and the gap between the armature and the magnetic yoke, affect the current at which the circuit breaker is tripped open.
The level of load current at which the circuit breaker trips varies with the feeder being protected and preference of the user. In order to accommodate a range of trip currents with a single trip unit, it is common to make the trip unit adjustable. A common range of adjustability of the trip current is five to ten times the rated current of the breaker. Typically, this adjustment is made in the spring force.
The trip unit must be calibrated at both the high and low end of trip currents to assure that the response is within tolerance, for example, plus or minus 20%. Calibration is effected by adjusting the gap between the spring biased armature and the magnetic yoke. As the magnetic yoke is fixed in position, the armature is moved, closer to the magnetic yoke to adjust the trip current downward, and away to increase the trip current. However, moving the armature in such a trip unit also changes the spring force, but in the opposite sense. Thus, while moving the armature closer to the magnetic yoke decreases the gap, tending to lower the trip current, it stretches the spring more, which increases the spring force, and therefore, the current needed to trip the breaker. In breakers with higher current ratings, the reduction in trip current produced by shortening the gap between the armature and the magnetic yoke is greater than the increase in trip current resulting from the associated stretching of the spring, so that the trip unit can be calibrated. However, in breakers with lower current ratings, the increase in trip current caused by the stretching of the spring is greater than the reduction caused by the shortening of the gap and, hence, the unit cannot be calibrated.
The present invention permits the gap between the armature and the magnetic yoke in a magnetic trip unit to be adjusted without affecting the bias force applied by the spring, thereby making it possible to calibrate such magnetic trip units regardless of the current rating. Thus, in accordance with one aspect of the invention, an adjustable magnetic trip unit for interrupting a load current through a circuit breaker comprises: a magnetic pole in which a magnetic field is generated by the load current; and an armature assembly. The armature assembly comprises a bracket supported for movement toward and away from the magnetic pole, a spring biasing the bracket to a position spaced from the magnetic pole, an armature, and a mount selectively positioning the armature on the bracket to adjust a gap between the armature and the magnetic pole. The adjustable magnetic trip unit can include in the armature assembly an adjustment mechanism selectively adjusting the bias applied by the spring to the bracket, whereby the load current, at which the magnetic field in the magnetic pole overcomes the bias applied by the spring and pulls the armature to the yoke, can be adjusted.
The mount for the armature can comprise a hinge connection and an adjustment member setting a hinge angle between the armature and the bracket. This adjustment member can comprise a threaded rod having a neck at one end. Either the armature or the bracket can have a tapped hole in which the rod is threaded while the other has a slot capturing the neck of the threaded rod.
The bracket can be elongated with a pivot member on at a first end, a first part of the hinge connection adjacent the first end and either the tapped hole or the slot adjacent the second end. In this configuration, the armature can comprise an armature paddle, an extension extending from the armature paddle and having a free end with the second part of the hinge connection adjacent the free end and with the other of either the tapped hole or the slot on the extension between the free end and the armature paddle. The bracket can have a T-shape at the first end forming the pivot member. Furthermore, the bracket can have a main body with an integral tab extending along each side toward the first end and offset from the main body to form the T-shape of the first end and configured to form the first part of the hinge connection. In this embodiment, the second part of the hinge connection on the extension of the armature comprises a T-shaped free end, and the tabs on the side edges of the bracket are configured as hooks forming the first part of the hinge connection on which the T-shaped free end of the armature extension seats.
The invention also embraces a circuit breaker incorporating this adjustable magnetic trip unit.